A particle beam therapy, which is a treatment of a deceased tissue by irradiating it with a particle beam to thereby damage the tissue, is a part of broad-sense radiation therapy. However, unlike a y-ray, an X-ray or like other ray, when a particle beam such as a proton beam, a heavy ion beam, etc., goes forward in a substance, its velocity decreases due to receiving resistance inversely proportional to the square of the velocity, and the beam rapidly stops after becoming a certain velocity or less, and at that time an imparted dose becomes highest. Accordingly, in the particle beam therapy, it is possible to control a range of irradiation in a depth direction by adjusting the velocity (energy) at the time of going into the body. Thus, in the particle beam therapy, it is possible to perform so-called layer-stacking conformal irradiation which is irradiation that makes the imparted dose corresponding to the three-dimensional shape of the deceased site in such a manner that the deceased site is partitioned in an in-body depth direction and an irradiation shape is formed for each partitioned layer (slice).
However, because of a relationship called as Bragg's curve between a depth and a dose distribution, when irradiation is made matching the peak (Bragg's peak) with a given slice, the imparted dose in a portion other than the slice becomes larger at the shallow side from the slice than at the deep side therefrom. Thus, when performing layer-stacking conformal irradiation, the irradiation time set for a shallow slice becomes shorter than the irradiation time set for a deep slice (see, for example, Patent Document 1). Meanwhile, in the operation of an accelerator that is fundamentally a beam source of the particle beam, although the particle beam is emitted by periodically repeating steps including injection, acceleration, emission and deceleration, the operation period is generally fixed. Thus, in a case where the operation period is matched with a shallow slice, when a deep slice is irradiated, the irradiation will be made extending over a plurality of the operation periods, so that a waiting time increases over the operation periods, resulting in extension of a treatment time. Instead, in a case where it is matched with a deep slice, when a shallow slice is irradiated, it is necessary to wait even after completion of the irradiation until completion of the operation period, resulting anyhow in extension of the treatment time.
Thus, it is considered to incorporate, for example, an operation method of a particle beam accelerator in which energy is changed in the middle of the operation period (see, for example, Patent Document 2), or a technique of changing an operation pattern (corresponding to the operation period) in the operation according to an amount of electric charge in the accelerator (see, patent Document 3).